Conventionally, acid and hydrogen peroxide, base and hydrogen peroxide, or an oxidation-reduction pair are employed as etching solutions for etching semiconductors. FIG. 17 is a cross sectional view illustrating a semiconductor device to which an etching is carried out using a prior art etching solution. In the figure, reference numeral 1 designates an n type AlGaAs layer. An n.sup.+ type GaAs layer 2 is disposed on the n type AlGaAs layer 1. A photoresist 3 is disposed on the n.sup.+ type GaAs layer 2. Reference character d designates an etching depth to which the n.sup.+ type GaAs layer 2 is etched and reference character .DELTA.d designates an over etching depth by which the n type AlGaAs layer 1 is excessively etched.
FIG. 18 is a cross sectional view illustrating an HEMT (high electron mobility transistor) produced by employing a prior art etching solution. In the figure, the same reference numerals as in FIG. 17 designate the same elements. Reference numeral 4 designates a GaAs substrate. An intrinsic (i type) GaAs layer 5 is disposed on the GaAs substrate 4. An i type InGaAs layer 6 is disposed on the i type GaAs layer 5. The n type AlGaAs layer 1 and the n.sup.+ type GaAs layer 2 are disposed on the i type GaAs layer 5 in FIG. 17. A drain electrode 7 and a source electrode 8 are disposed on the n.sup.+ type GaAs layer 2.
The threshold voltage V.sub.th of this HEMT is represented by the following formula (1): ##EQU1## where .phi..sub.B : Schottky barrier height
W: thickness of AlGaAs layer PA1 N.sub.D : donor concentration in AlGaAs layer PA1 .DELTA.E.sub.c : energy discontinuity of the conduction band between AlGaAs and GaAs PA1 .epsilon.: dielectric constant of AlGaAs
A description is given of the prior art etching method.
When a solution of phosphoric acid, hydrogen peroxide, and water in a volume ratio of 3:1:50 is employed as an etching solution in FIGS. 17 and 18, the etching speeds of the n.sup.+ type GaAs layer 2 and the n type AlGaAs layer 1 are both about 30 nm/min., and therefore, when this etching solution is applied to producing a gate recess of HEMT, there arises a variation .DELTA.d in the recess depth (d) in FIGS. 17 and 18.
Since the thickness W of the AlGaAs layer 1 which appears in the formula (1) defining the threshold voltage V.sub.th of the HEMT, is determined by the recess depth (d), the thickness W of the AlGaAs layer 1 varies when the recess depth (d) varies as described above, resulting in a variation in the threshold voltage V.sub.th and in the characteristics of the HEMT.
The prior art etching solution does not have a selectivity for different materials as described above, and when it is used in producing a heterostructure device, precise control was difficult.